In the field of refrigerating plants there has always been a need for regulating the refrigerating capacity of the compressor based on the actual thermal load required. In fact, it often occurs that the refrigerating capacity of the compressor is excessive compared to the required thermal load. Such situation occurs, for example, in supermarket refrigerating plants or refrigerating rooms during night hours. The thermal load required overnight is, in fact, definitely lower than the thermal load required during daily hours, wherein operations of massive removal of food from refrigerating rooms, opening of freezer doors in a crowded place, etc. occur.
If the refrigerating capacity of the compressor is constant, during night hours the compressor is capable of bringing the environment to the desired temperature in a shorter time than the times required during daily hours.
Such situation may determine the occurrence various shortcomings, which contribute to compromising the efficiency of the entire refrigerating plant.
For example, the excessive refrigeration may lead to the formation of ice on the evaporator and determine a lowering of the dew point of the evaporator.
The lowering of the dew point may cause an excessive dehumidification of the processed air, creating uncomfortable conditions if the compressor supplies an air-conditioning system, or may cause a significant weight drop of the product stored if the compressor supplies a refrigerating room.
Some known solutions provide for the switching off of the compressor upon reaching the desired temperature and the restart of the compressor when the temperature exceeds a predetermined threshold value. Such solution, however, risks to dangerously increase the number of start-ups per hour of the compressor and significantly reduce the operating life of the compressor.
Another known solution provides for a plurality of low-power compressors instead of a single high-power compressor. However, such solution, besides having a high starting cost and a more complicated control system, does not overcome the disadvantage of having to frequently restart the compressors to face the thermal load variation.
A further known solution provides for the complete closing of the suction channel of the refrigerating fluid of at least one of the pistons of the plurality of compressor pistons when a reduction of thermal load occurs. In particular, such solution provides the use of a solenoid valve, which closes the suction channel when the evaporation temperature and pressure decrease below a respective threshold value. In this way, the “pumping” effect of at least one of the plurality of compressor pistons is cancelled and the flow rate of the compressed refrigerating fluid is reduced, reducing the refrigerating capacity of the compressor. Such solution, however, allows to reduce the input flow rate by 50% at the utmost when the supply channel of one piston out of two is closed. This type of regulation is too rough and is not sufficient to optimize the performance and efficiency of the plant upon variation of the required thermal load.
In order to obviate this drawback, a control system of the solenoid valve is used which is configured to time the opening of the solenoid valve in order to obtain a desired reduction of the flow rate of refrigerating fluid input to the piston. A control system of this type is described in the document US 2006/0218959. However, such solution proved to be not very efficient in that it determines a continuous fluctuation of pressures and temperatures inside the compressor, subjecting the inner parts of the compressor to dangerous stresses. Therefore, such solution is not reliable and requires constant monitoring of the operating conditions of the compressor.